A wide-band multi-mode fiber, like the OM4 multi-mode fiber, is understood to be a multi-mode fiber having an exemplary wavelength range between 850 nanometers and 950 nanometers.
Multi-mode fibers are successfully used in high-speed data networks together with high-speed sources that typically use transversely multimode vertical cavity surface emitting lasers, more simply called VCSELs. However, multi-mode fibers are affected by intermodal dispersion, which results from the fact that, for a particular wavelength, several optical modes propagate simultaneously along the fiber, carrying the same information, but travelling with different propagation velocities. Modal dispersion is expressed in term of Differential Mode Delay, DMD, which is a measure of the difference in pulse delay between the fastest and slowest modes traversing the multi-mode fiber.
In order to minimize modal dispersion, the multi-mode optical fibers used in data communications generally include a core having a refractive index that decreases progressively going from the center of the fiber to its interface with a cladding (e.g., a typical multi-mode fiber has a graded-index profile).
When a light signal propagates in such a core having a graded index, the different modes experience a different propagation medium, which affects their speed of propagation differently. It is theoretically possible to obtain a group velocity that is virtually equal for all the modes and thus a reduced intermodal dispersion for a particular wavelength.
Enabled by VCSEL technology, high-speed multi-mode optical fibers, such as OM4 fibers (which are laser-optimized, high bandwidth 50 μm multi-mode fibers, standardized by the International Standardization Organization in Publication IEC 60793-2-10:2015 (Edition 5), published Nov. 19, 2015, fiber type Ala.3), have proved to be the medium of choice for high-data-rate communications, delivering reliable and cost-effective 10 to 100 Gbps solutions. The combination of Wide-Band (WB) multi-mode fibers with longer wavelengths VCSELs for Coarse Wavelength Division Multiplexing (CWDM) may be an interesting option to be considered in order to meet the future increase of demand. Publication IEC 60793-2-10:2015 is hereby incorporated by reference in its entirety.
However, the modal bandwidth of, for example, OM4 fibers has until now been achieved only over a narrow wavelength range, typically 850 nanometers+/−10 nanometers. The feasibility of Wide-Band, WB, multi-mode fibers satisfying OM4 performance requirements over a broader wavelength range is a challenge to overcome for next generation multi-mode systems.
Multi-mode fiber performance is typically defined by an assessment of Effective Modal Bandwidth, EMB, at a given wavelength. For example, OM4 fibers should exhibit EMB greater than 4,700 MHZ-km at a wavelength of 850 nm+/−1 nanometers. The achievement of such high EMB values requires extremely accurate control of refractive index profile of multi-mode fibers. Until now, traditional manufacturing processes cannot guarantee such high EMB, and it is difficult to accurately predict the EMB values from refractive index profile measurements on core rod or cane. This is especially so when high EMB values, typically larger than 2,000 MHz-km, are expected, meaning the optical fiber's refractive index profile is close to the optimal profile. As a matter of fact, EMBs are directly assessed on the fibers.
A few-mode fiber is typically defined by Differential Mode Group Delays, DMGDs. The DMGDs are measured using a DMD technique, mostly at a wavelength of 1550 nanometers. Other wavelengths might also be of future interest once few-mode fibers, FMFs, are used in wide band applications as well.
The Effective Modal Bandwidth, EMB, is assessed by a measurement of the delay due to the modal dispersion, known under the acronym DMD for “Differential Mode Delay.” It includes recording pulse responses of the multi-mode fiber, or the few-mode fiber, for single-mode launches that radially scan the core. It provides a DMD plot that is then post-processed in order to assess the minimal EMB a fiber can deliver. The DMD measurement procedure has been the subject of standardization and is specified by the International Standardization Organization in Publication IEC 60793-1-49:2006 (Edition 2), published Jun. 26, 2006, which is hereby incorporated by reference in its entirety. The DMD metric, also called DMD value, is expressed in units of picoseconds per meter (ps/m). It assesses the delay between the fastest and the slowest pulses considering a collection of offset launches normalized by fiber length. It basically assesses a modal dispersion. Lower DMD value (i.e., lower modal dispersion as measured by DMD) generally results in higher EMB.
The DMD measurement procedure includes measuring the fiber response when a pulse, or a pulse train, is launched with a fiber that is single mode at the wavelength of interest. The excited modes in the multi-mode fiber or the few-mode fiber, (i.e., the fiber under test, FUT) depend on the lateral position of the single mode fiber with respect to the FUT optical axis. Basically, centered launch excites the lowest order modes while offset launches excite the highest order modes. Therefore, a collection of records of fiber response when the single mode fiber scans the core of the FUT gives a good overview of the modal dispersion of the FUT. It is noted that DMD measurements typically require an alignment procedure to allow correct centered launch (i.e., when the single mode probe fiber axis is aligned with the FUT optical axis).
A known measurement arrangement includes a laser that is arranged to emit a train of laser pulses of a few picoseconds up to hundreds of picoseconds at a single wavelength. The laser pulse is coupled into a single mode fiber via a first component comprising, for example, mirrors and/or optics. The single mode fiber is coupled to the FUT via a second component, which comprises a translation stage allowing a lateral translation of the single-mode fiber with respect to the FUT fiber optical axis. The output of the FUT is coupled into a detector module, via a third component. The detector module is arranged to convert the optical waveform into an electrical waveform. The detector module is further arranged to sample the received electrical waveform and to allow signal recording. Until recently, DMD measurements were performed at a single wavelength.
Nowadays, wavelength division multiplexing, WDM, is expected to be used in data communication systems in order to extend the multi-mode fiber capacity. For instance, four channels at 10 Gbps or 40 Gbps spaced within the 850-950 nanometer range are going to be used to deliver 100 Gbps or 400 Gbps through a single multi-mode fiber. As a consequence, DMD measurements over the whole 850-950 nanometer range are necessary to assess the modal dispersion of multi-mode fibers.
In addition, single mode fibers are likely to be replaced by few-mode fibers in regular optical communications in order to enlarge the fiber capacity through spatial mode multiplexing. The modal dispersion of the few-mode fibers is also relevant, and so the need for accurate DMD measurements at several wavelength is increasing rapidly.
U.S. Patent Publication No. 2014/0368809, which is hereby incorporated by reference in its entirety, discloses a differential mode delay measurement system for an optical fiber. The system includes an optical test fiber with a plurality of modes, a single mode light source that provides a continuous light wave signal to a modulator, and a pulse generator that provides an electrical pulse train signal to the modulator and a triggering signal to a receiver. The modulator is configured to generate a modulated optical test signal through the optical fiber based at least in part on the received light wave and pulse train signals, and the receiver is configured to receive the test signal transmitted through the fiber and evaluate the test signal based at least in part on the triggering signal.
European Patent Publication No. 1,705,471 and its counterpart U.S. Pat. No. 7,369,249, each of which is hereby incorporated by reference in its entirety, disclose an apparatus for measuring the differential mode delay of multi-mode optical fibers. The apparatus includes a tunable laser source, an interferometer, a data-collecting device, and a computer. The tunable laser source outputs light, frequencies of which vary linearly. The interferometer generates multi-mode light and single mode light by separating light, which is output from the tunable laser source, transmitting the multi-mode light and the single mode light to the multi-mode optical fiber, which is a measurement target, and a single mode path, which is a reference, and generating a beating signal by causing the multi-mode light and the single mode light to interfere with each other.